The invention relates to a cutting insert for the machining of workpieces, especially the roughing-finishing machining of gray cast iron by surface milling and wherein the rake surface is bounded by cutting edges of which two mutually-adjacent cutting edges form a cutting corner, the cutting insert having at least one trough-shaped indentation interrupting the cutting edges.
The invention relates further to a tool comprised of a tool holder with at least one recess for receiving such a cutting insert.
Milling machining is generally carried out in two working steps, a premilling (roughing) and a fine milling. A combined roughing-finishing tool can be provided which has a part of the cutting inserts clamped in the recesses (seats) provided in the tool holder utilized as purely roughing tools and the remainder utilized as finishing tools. Examples for finishing cutting inserts are described in DE 197 03 569. The cutting inserts there described, which preferably have rake faces as regular hexagons, comprise a plurality of main cutting edges and auxiliary cutting edges connected thereto. As has been described in this publication, a cutting insert of that type can be used in combination with a cutting insert for roughing.
The cutting inserts described in DE 197 03 569 A1 have in conjunction with the auxiliary cutting edges a concave chip-forming groove whose radius is between 0.5 mm and 0.8 mm. With this chip-forming groove the cutting angle of the auxiliary cutter should be set between 10xc2x0 and 20xc2x0. A chip-forming groove also extends along the main cutting edges and in cross section has a radius of curvature of 0.5mm measured in cross section. This publication does not describe purely roughing turning inserts.
A cutting insert is known from DE 28 40 610 C2 which has indentations in the rake face along the cutting edges and of a width whose greatest dimension is parallel to the cutting edges and which is greater than the distance between two neighboring indentations. Each of these indentations interrupts the cutting edge which here has a shape which changes in the region of the interruption, i.e. no longer forms a straight line, but rather in the region of the interruptions has a configuration rearwardly of the straight line cutting edge parts in the chip travel direction. Because of these features, buckling is produced in the chip travelling away from the cut which serves to break up the chip into short pieces. The region of the cutting corner should however be free from indentations and instead should have a planar rake surface portion or a negative consolidation surface to avoid an increase in the tendency toward buckling of the chip in the corner region. The mentioned indentations can, according to DE 28 40 610 C2, also be arranged in a chip-forming trough running along the cutting edge. In these publications a trigonally-shaped cutting insert is described which is unsuitable for rough milling operations.
U.S. Pat. No. 4,710,069 describes a cutting insert which has along its cutting edges a chamfer and in the region of the chamfer remote from the cutting edge, a groove-like recess. The boundary line between the mentioned chamfer and the groove-like recess is interrupted by a multiplicity of equally spaced recesses whose depths should be less than 0.1 mm. The recesses are substantially partially spherical recesses with a radius of curvature of 0.5 mm. Such cutting inserts are however only suitable for turning-type machining operations.
The shapes of roughing-finishing inserts which have been provided up to now have respective chip-forming troughs in conjunction with the cutting edges and extending over their entire lengths. In the region of a cutting edge corner, this chip-forming trough is interrupted by relatively higher lying stiffening surfaces. The roughing-finishing machining is however carried out with cutting depths of approximately 0.5 mm to 1 mm. Thus the rounded cutting corner effects cutting so that a modification of the chip-forming groove to increase the chip-forming angle does not yield any effective improvement. Moreover, a lengthening of a chip-forming groove in the cutting corner region while reducing the width of the stiffening surfaces readily reaches limits since remaining sharp edged projections, especially in the cutting corner region, tend to crumbling which significantly reduces the life of the cutting insert.
It is thus an object of the present invention to provide a cutting insert for roughing-finishing and with which the axial forces arising in milling use are significantly reduced.
This object is achieved, according to the invention, by a cutting insert in which the indentations are arranged in the cutting-corner regions and interrupt both of the cutting edges which adjoin there.
In contrast to the teaching disclosed in DE 28 40 610 C2, to the effect that the cutting corner region should be left free from trough-like depressions or indentations, the cutting inserts formed according to the invention show significantly reduced cutting forces and a resulting improved life by comparison with the previous cutting inserts. Through the use of a spherical segmental recess in the cutting corner regions, significantly larger chip angles are permitted and for a cutting insert clamped in a tool holder greater effective radial and axial chip angles can be realized. The cutting edges which are linear except in the cutting corner regions are, as a result of the indentations, concavely curved at their rear parts in the direction of the cutting corners and the cutting corners lie approximately at the height or level of the linearly running cutting edge or slightly therebelow. This is repeated in a corresponding manner whereever the cutting edges adjoin the cutting corners. The cutting insert is used in milling in that preferably the cutting edge portions falling away in a concave manner at both sides of the cutting corners are effective for cutting purposes. The maximum width of the indentations in the cutting corner region depends upon the desired cutting depth. In the region of the cutting corner which is effective for the cutting operation, a trough can be formed which allows the positive cutting angle to be optionally selected.
Preferably the indentations are symmetrical to the angle bisectors of the cutting corners.
According to a further feature of the invention, the indentations are formed in part as spherically segmental, whereby the radius of curvature of the concave indentations lies between 4 mm and 10 mm. The maximum depth of the indentations should preferably not be greater than 0.4 mm to 0.6 mm.
Since the cutting corners should only be used to an effective cutting depth of 0.5 mm to 1 mm, the maximum diameter of the indentation should be 4 mm.
To maintain a sufficient cutting corner stability, the cutting angle xcex3 should lie between +5xc2x0 and +20xc2x0 in the cutting corner region, preferably between +8xc2x0 and +15xc2x0.
So that the cutting insert can be used not only on one side but also on the top surface as well as the bottom surface, a central rake face plateau can be provided for two-sided support, the cutting edge planes projecting over the rake surface plateau. The intermediate cutting surface plateau has, according to a further feature of the invention, nose-shaped projections which extend into the rearward region of the indentations.
Preferably the cutting inserts of the invention are of regular six-corner form in which the respective mutually adjacent cutting edges are oriented at an obtuse angle of 120xc2x0. The cutting edges are rounded so that, according to a feature of the invention, the cutting corner rounding has a radius between 0.4 mm and 3 mm. As has already been indicated, the cutting insert is configured to be useful on both sides so that twelve usable cutting corners are provided for machining.
The cutting insert according to the invention is preferably received in a recess or seat provided therefor in a tool holder configured as a milling tool body and are fastened in the recess. For fastening, clamping wedges can be used for fixing the cutting insert with respect to the tool axis. It is, however, also possible to arrange the cutting inserts in cassettes which can then be affixed in the tool carrier in the desired orientations. According to the invention, an orientation of the cutting insert for roughing is selected in which it has an effective axial cutting angle xcex3p of +5xc2x0 to +10xc2x0 and/or an effective radial cutting angle of 0xc2x0 to +8xc2x0 in the cutting insert recess.